EPR/Spin-trapping study of free radical intermediates in the photolysis of trifluoromethyl ketones with initiators

Article abstract of DOI:10.1002/mrc.2566

Photolysis of trifluoromethyl ketones (TFMKs) 1a-1e versus the non-fluorinated ketones 2a-2b in the presence of radical initiators by electron paramagnetic resonance spectroscopy has been studied for the first time. The transient radicals generated after irradiation of the ketones were identified by trapping with 2-methyl-2-nitrosopropane (MNP) and 2,4,6-tritert- butylnitrosobenzene (TTBNB) as spin traps. TTBNB is a powerful, particularly useful spin trap in these kinds of processes producing anilino and nitroxyl spin adducts due to the ambivalent reactivity on the N and O atoms. In the presence of t-butylperoxide, short-chain TFMKs, such as 1,1,1-trifluoroacetone (1d) and hexafluoroacetone (1e), give rise to detection of the elusive trifluoromethyl radical. In contrast, long-chain TFMKs did not provide clues to prove formation of the trifluoromethyl radical but instead to radicals derived by abstraction of one α-methylene proton to the carbonyl. Although TFMKs are quite stable to photodegradation in the absence of initiator, methyl ketone 2b and phenyl ketone 3 produce radicals resulting from abstraction of a γ-hydrogen to the carbonyl group. Copyright

Full text of DOI:10.1002/mrc.2566

Research Article
Received: 3 July 2009
Revised: 3 November 2009
Accepted: 9 December 2009
Published online in Wiley Interscience: 7 January 2010
(www.interscience.com) DOI 10.1002/mrc.2566
EPR/Spin-trapping study of free radical
intermediates in the photolysis of
trifluoromethyl ketones with initiators
a
∗
`
Esmeralda Rosa, Angel Guerrero, M Pilar Bosch and Luis Julia
Photolysis of trifluoromethyl ketones (TFMKs) 1a–1e versus the non-fluorinated ketones 2a–2b in the presence of radical
initiators by electron paramagnetic resonance spectroscopy has been studied for the first time. The transient radicals
generated after irradiation of the ketones were identified by trapping with 2-methyl-2-nitrosopropane (MNP) and 2,4,6-tri-
tert-butylnitrosobenzene (TTBNB) as spin traps. TTBNB is a powerful, particularly useful spin trap in these kinds of processes
producing anilino and nitroxyl spin adducts due to the ambivalent reactivity on the N and O atoms. In the presence of
t-butylperoxide, short-chain TFMKs, such as 1,1,1-trifluoroacetone (1d) and hexafluoroacetone (1e), give rise to detection of the
elusive trifluoromethyl radical. In contrast, long-chain TFMKs did not provide clues to prove formation of the trifluoromethyl
radical but instead to radicals derived by abstraction of one α-methylene proton to the carbonyl. Although TFMKs are quite
stable to photodegradation in the absence of initiator, methyl ketone 2b and phenyl ketone 3 produce radicals resulting from
c
abstraction of a γ -hydrogen to the carbonyl group. Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Keywords: irradiation; spin trap; spin adduct; EPR; trifluoromethyl ketones; photolysis
Introduction
reactions.[5–7] Some of the TFMKs to study, particularly those
structurally related to insect sex pheromones, have shown
their effectiveness as potential control agents of economically
important pests.[8] We describe herein for the first time the
stability of TFMKs 1a–1e (Fig. 1) by irradiation in the presence
of hydrogen peroxide or di-tert-butyl peroxide as initiators. Two
nitroso compounds, 2-methyl-2-nitrosopropane (MNP) and 2,4,6-
tri-tert-butylnitrosobenzene (TTBNB), are used as spin traps of
the transient radicals generated in the irradiation processes. The
results have been compared with the non-fluorinated ketones
2a–2b and 3. An important goal of the study was to disclose
whether the TFMKs generate the highly elusive trifluoromethyl
radicals under the irradiation conditions.
The spin-trapping technique can be used to uncover radical
mechanisms of many organic reactions through electron param-
agnetic resonance (EPR). This technique is particularly useful in
processes in which only low steady-state radical concentrations
are present.[1–3] Free radicals, as reactive transient intermediates
in many organic reactions, are scavenged by spin traps, such as
C-nitroso compounds and nitrones, and converted to more sta-
ble spin adducts. EPR spectra of these adducts provide valuable
information about the structure of the corresponding radical inter-
mediates and therefore to unravel the mechanism of the reactions
in which they are involved. Nitroso spin traps have the advantage
that the reactive radical attaches directly to the nitroso nitrogen
atom, and itis therefore in close proximity to the unpaired electron
that is located primarily in the nitroxyl function. This usually results
in the detection of additional distinctive hyperfine couplings from
magnetic nuclei present in the added radical. In nitrone spin traps,
however, the reactive radical adds to the carbon atom adjacent
to the incipient nitroxyl group, and it is therefore more distant
from the molecular orbital containing the unpaired electron. So, it
is often not possible to resolve any hyperfine couplings from the
added radical itself. This adduct identification is easier and more
definitive when using nitroso spin traps than nitrones.
In the course of our work directed to the inhibition of
the chemical communication in insects as a new biorational
approach for pest control,[4] we needed to know the stability
of trifluoromethyl ketones (TFMKs) in photochemical conditions
and in the presence of initiators. It is well known that nucleophilic
radicals such as hydroxyl and alcoxyl radicals abstract hydrogen
from an α-C–H group in a ketone. Polar effects from the carbonyl
group stabilize an unpaired electron on an adjacent carbon atom
and, consequently, play an important role in determining the
chemo- and regio-selectivity of these hydrogen atom transfer
Results and Discussion
Irradiation of ketones: identification of radicals formed in the
process
Two spin traps, MNP and TTBNB, were selected to scavenge
transient radicals formed in the irradiation of ketones displayed in
Fig. 1. Bothcompoundspreferentiallytrapcarbon-centredradicals
R•. When MNP is used as spin trap, R• attacks the nitrogen atom of
the nitroso group to form nitroxyl adducts of type (a) (Scheme 1).
A general drawback observed with the use of MNP as spin
trap is the presence of an intense and very persistent EPR signal
∗
`
Correspondence to: Luis Julia, Department of Biological Chemistry and
Molecular Modellization, IQAC (CSIC), Jordi Girona, 18-26, 08034 Barcelona,
Spain. E-mail: ljbmoh@cid.csic.es
Department of Biological Chemistry and Molecular Modellization, IQAC (CSIC),
Jordi Girona, 18-26, 08034 Barcelona, Spain
c
Magn. Reson. Chem. 2010, 48, 198–204
Copyright ꢀ 2010 John Wiley & Sons, Ltd.

EPR/Spin-trapping study of free radical intermediates
O
O
CF3
CF3
CF3
CH3
1a
1b
O
O
O
O
CF3
CF3
F3C
2a
1c
1d
1e
O
O
CH3
Ph
2b
3
Figure 1. Structures of the ketones considered for the photodegradation study.
proceeded first to study the transient radicals resulting from
irradiation of more simple fluorinated ketones, such as 1,1,1-
trifluoroacetone (1d) and hexafluoroacetone (1e), and the non-
fluorinated acetone, 2-heptanone (2a), 2-undecanone (2b) and
tridecylphenyl ketone (3). The hyperfine splittings of all radical
adducts were easily derived by simulation[13] and are summarized
in Tables 1 and 2.
O
+
O
t-BuN
(MNP)
Scheme 1. Radical adduct from MNP as spin trap.
R
t-BuNR
(a)
hυ
Using MNP as spin trap
t-BuNO
t-BuNO
t-Bu +
NO
t-Bu2N
When ketones 1d, 1e and acetone were irradiated in the presence
of MNP and in the absence of a hydrogen scavenger only a very
strong triplet corresponding to the di-tert-butylnitroxide radical
(I) was observed. However, in the presence of hydrogen peroxide
as initiator, irradiation of acetone showed a new spin adduct in
addition to radical I. This new radical (aN = 14.90 G; a2H = 8.70 G;
g = 2.0059 0.0005) was assigned as adduct II as previously
described,[15] and its formation can be explained by abstraction of
one hydrogen atom of acetone by the hydroxyl radical generated
by photolysis of hydrogen peroxide, followed by reaction with
MNP (Scheme 4).
When 1,1,1-trifluoroacetone (1d) was irradiated in the presence
of hydrogen peroxide, abstraction of a hydrogen in the α po-
sition to the carbonyl yielded the trifluoroacetylmethyl radical
(•CH2COCF3), which could decompose to ketene and trifluo-
romethylradical.ThislatterandelusiveradicalwastrappedbyMNP
to generate spin adduct III (Scheme 5). Trifluoromethyl radical was
first trapped from photolysis of trifluoroiodomethane in benzene
in the presence of phenyl-tert-butylnitrone (PBN).[14,16] A similar
O
+
t-Bu
I
Scheme 2. Radical adduct from photolysis of MNP.
O
ArNO + R
ArNR + ArN
(b)
OR
(c)
t-Bu
t-Bu
Ar:
t-Bu
Scheme 3. Radical adducts from TTBNB as spin trap.
Table 1. EPR spectral parameters of spin adducts generated by
irradiation of ketones 1a, 1c, 1d, 1e, 2a, 2b in the presence of
hydrogen peroxide and MNP as spin trap
corresponding to the di-tert-butylnitroxide radical (I), resulting
from reaction of the initially formed tert-butyl radical with
unreacted MNP[9] (Scheme 2). Radical I appears as a triplet
corresponding to the N hyperfine splitting (aN = 16.00 G), often
masking the signals of other expected spin adducts (Scheme 2).
TTBNB is also an excellent spin trap.[10] It has two trapping
sites: the nitrogen atom to form nitroxyl radical as spin adduct
(b) and the oxygen to form the anilino adduct (c) if R• is bulky[11,12]
(Scheme 3). Generally, primary alkyl radicals form nitroxides (b),
tertiary radicals give rise to anilino radicals (c), and secondary
radicals may produce both types of radicals.
Radical
Ketones adducts
Spectral parameters
1a
1c
1d
1e
2a
2b
VI
aN = 15.40 G; a2H = 2.40 G; g = 2.0067 0.0005
VII aN = 15.50 G; a2H = 2.60 G; g = 2.0060 0.0005
IIIa aN = 13.50 G; a3F = 12.90 G; g = 2.0060 0.0005
IIIa aN = 13.50 G; a3F = 12.90 G; g = 2.0060 0.0005
IV
V
aN = 14.80 G; a2H = 2.00 G; g = 2.0058 0.0005
aN = 15.00 G; a2H = 2.00 G; g = 2.0063 0.0005
In order to study systematically the nature of the spin adducts
formed in the irradiation of TFMKs 1a–1c in the presence of
a hydrogen scavenger and MNP or TTBNB as spin traps, we
a Parameters from literature[14] do not distinguish between N- and
F-couplings (aN = a3F = 12.24 G in acetone).
c
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E. Rosa et al.
Table 2. EPR spectral parameters of spin adducts generated by
irradiation of ketones 1a, 1b, 1d, 1e, 2b and 3 in the presence of
hydrogen peroxide in acetone or di-tert-butyl peroxide (neat) and
TTBNB as spin trap
Radical
Ketones adducts
Spectral parameters
1a
1b
1d
1e
2b
3
XV aN = 11.13 G; a3H = 1.75 G; g = 2.0048 0.0005
XVI aN = 13.75 G; a1H = 21.75 G; g = 2.0063 0.0005
XVII aN = 10.75 G; a3H = 1.88 G; g = 2.0048 0.0005
XVIII aN = 13.75 G; a1H = 23.25 G; g = 2.0066 0.0005
IX
aN = 10.25 G; a3F = 9.4 G; a2H = 0.75 G;
g = 2.0069 0.0005
IX
aN = 10.25 G; a3F = 9.4 G; a2H = 0.75 G;
g = 2.0069 0.0005
X
aN = 10.75 G; a3H = 1.88 G; g = 2.0046 0.0005
aN = 13.88 G; a1H = 23.13 G; g = 2.0061 0.0005
XI
XII aN = 10.63 G; a3H = 1.88 G; g = 2.0047 0.0005
XIII aN = 13.75 G; a1H = 23.25 G; g = 2.0060 0.0005
result was obtained upon irradiation of hexafluoroacetone (1e).
It is worthwhile to note that for the first time the trifluoromethyl
radical has been trapped with MNP by irradiation of TFMKs in
solution and in the presence of radical initiators. The hyperfine
splittings of radical III are depicted in Fig. 2 and Table 1.
Irradiation of neat 2-heptanone (2a) and 2-undecanone (2b)
in the presence of hydrogen peroxide and MNP yielded two new
radicalsintheformofatripletofdoublets,whichwereattributedto
spinadductsIVandV, respectively, inadditiontoradicalI(Table 1).
In these new ketones, the hydroxyl radical preferentially abstracts
one methylene hydrogen in the α-position to the carbonyl over
one of the methyl group, in accordance with previous results[15]
(Scheme 6).
Figure 2. (a) EPR spectrum of spin adduct III after irradiation of ketones
1d and 1e with hydrogen peroxide as initiator and MNP as spin trap.
(b) Computer simulation of III using the parameters given in Table 1.
Similarly, when TFMKs 1a and 1c were irradiated in acetone in
the presence of hydrogen peroxide and MNP, we detected spin
adducts VI and VII, respectively, by abstraction of one methylene
protonintheα-positiontothecarbonylbytheinitiator.Inaddition,
adducts I and II from the MNP and the acetone present in the
sample were also detected (Scheme 7, Table 1). It is worth noting
that in no case, however, the spin adduct III corresponding to
trapping of the CF3• radical was detected.
hυ
2HO
H2O2
CH3COCH3
+
CH2COCH3
O
HO
t-BuNO + CH2COCH3
t-Bu
N
CH2COCH3
II
O
N
O
N
Scheme 4. Photolysis of acetone in the presence of H2O2 and MNP as spin
trap.
CHCOCH3
(CH2)3CH3
t-Bu
CHCOCH3
(CH2)7CH3
t-Bu
IV
V
hυ
2HO
H2O2
Scheme 6. Radical adducts from photolysis of 2-heptanone (2a) and 2-
undecanone (2b) in the presence of H2O2 and MNP as spin trap.
CH3COCF3 + HO
CH2COCF3
1d
O
CH2COCF3
+
CF3
CH2
C
O
hυ
I + II
+
N
CHCOCF3
R
t-Bu
1a, 1c
MNP, H2O2
acetone
O
N
CF3 + t-BuNO
t-Bu
CF3
R = (CH2)9CH=CH(CH2)3CH3
VI
R = (CH2)7CH3 VII
III
Scheme 5. Photolysis of 1,1,1-trifluoroacetone in the presence of H2O2
Scheme 7. Photolysis of trifluoromethylketones 1a and 1c in the presence
and MNP as spin trap.
of H2O2 and MNP as spin trap.
c
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EPR/Spin-trapping study of free radical intermediates
in these spectra is surprising. No spectral signals appeared in the
absence of the initiator.
Photolysis of neat (absence of solvent) ketones 2b and 3 in
the presence of di-tert-butyl peroxide as radical initiator allowed
detection of the pairs X, XI and XII, XIII, respectively, along with
the simple adduct XIV in both processes (Scheme 9). Adducts X,
XI, XII and XIII were formed by trapping the transient radicals
derived from abstraction of one α-methylene hydrogen to the
carbonyl group of the ketones, whereas adduct XIV resulted by
trapping the hydrogen atom.[17] As an example, the EPR spectrum
of the adducts detected in the photolysis of ketone 2b is shown in
Fig. 4. The spectral parameters of radicals are shown in Table 2.
These experiments allowed us also to establish the higher
stability of the nitroxyl adducts with regard to the anilino adducts.
Thus, although the predominant spin adducts first generated
upon irradiation were those of an anilino nature (X, XII), when the
solutions were left in the dark for 24 h more, the only detected
adducts were then the corresponding nitroxides XI, XIII (Fig. 4).
Similarly, when fluorinated ketones 1a and 1b were irradiated
in the presence of di-tert-butyl peroxide and TTBNB a mixture of
anilino and nitroxyl radicals, assigned as XV and XVI from ketone
1a and XVII and XVIII from ketone 1b, respectively, were detected
(Scheme 10). The radicals were again assigned by their splittings
and g-values (Table 2) and only the nitroxyl adducts were stable
in the dark after 24 h, in a similar manner to the irradiation of the
non-fluorinated ketones 2b and 3.
The different behavior in the photodegradation in the presence
of an initiator of acetone 1d and ketones 1a, 1b and 1c could
proceed from the differences in the relative stability of the radical
intermediates generated by an αH-abstraction reaction, that is
a primary carbon radical in 1d and a secondary radical in the
others. In acetone 1d, the intermediate methyl radical leads by an
intramolecularelectronicrearrangementtotrifluoromethylradical
which is scavenged by the spin trap to give the final spin adduct,
as shown in Scheme 5. In ketones 1a, 1b and 1c, the intermediate
methylene radicals are stable enough to give the expected spin
adducts.
Figure 3. (a) EPR spectrum of spin adduct IX from irradiation of ketones
1d and 1e with hydrogen peroxide as initiator and TTBNB as spin trap.
(b) Computer simulation using the parameters given in Table 2.
O
N
t-Bu
O
N
Ar
CF3
Ar
CH2COCH3
VIII
t-Bu
Ar =
t-Bu
IX
Photolysis in the cavity of the EPR spectrometer of ketones
containing γ -hydrogens to the carbonyl could produce the spin
adductsresultingfromtrappingtheradicalsderivedbyabstraction
of a γ -hydrogen, according to a Norrish type II mechanism.[18]
However, photolysis of TFMKs 1a and 1b in the absence of
initiator and in the presence of TTBNB gave no detectable EPR
signals except those of adduct XIV. In the same way, irradiation
of ketone 2b under identical conditions did yield the nitroxide
XIX (aN = 13.75 G; a1H = 22.75 G; g = 2.0067 0.0005) by
abstraction of a γ -hydrogen of the ketone and carbonyl reduction
to the secondary alcohol (Fig. 5, Scheme 11). In the same line,
irradiation of the aromatic ketone 3 led to both anilino- and
nitroxyl adducts XX (aN = 10.60 G; a2H = 1.75 G; a1H = 1.0 G;
g = 2.0048 0.0005) and XXI (aN = 14.0 G; a1H = 23.25 G;
g = 2.0060 0.0005), respectively, following analogous Norrish
type II degradation (Fig. 5, Scheme 11). In both cases, radical XIV
appeared also superimposed on the main radical adducts. In no
case radicals derived from cleavage of the α-CC bonds of the
carbonyl were trapped (Norrish type I mechanism).[18]
Scheme 8. Radical adducts from photolysis of acetone (radical adduct VIII)
and of ketones 1d and 1e (radical adduct IX) in the presence of H2O2 and
TTBNB as spin trap.
Using TTBNB as spin trap
As cited above, this nitroso compound gives rise to two different
kinds of spin adducts, the nitroxyl and anilino adducts[10,12] and
bothcanbedistinguishedbytheirg-valuesandnitrogensplittings.
Usually, the g-values of the anilino adducts are slightly lower
than those of the corresponding nitroxyl adducts. An important
advantage of TTBNB over MNP is its photostability: the former is
transparent in EPR upon irradiation in benzene. Unfortunately, this
solvent could not be used in our experiments due to the partial
solubility of some of the substrates, particularly TFMKs 1a and 1b.
Long irradiation (∼40 min) of TTBNB in acetone in the presence
of hydrogen peroxide as radical initiator led to the detection of
spin adduct VIII (aN = 13.94 G; a2H = 14.52 G; a1H = 1.0 G;
g = 2.0058 0.0005), derived from reaction of the acetylmethyl
radical with the spin trap (Scheme 8). The spectral parameters
of VIII (Table 2) are in close agreement with those reported
previously.[12] Irradiation of ketones 1d and 1e in acetone in the
presence of the initiator and TTBNB yielded indistinctly radical
IX by trapping, as with MNP, the nascent trifluoromethyl radical
(Scheme 8) (Fig. 3, Table 2). The absence of the anilino adducts
Conclusions
In the presence of a radical initiator (hydrogen peroxide or di-tert-
butyl peroxide), irradiation of TFMKs and methyl ketones give
c
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E. Rosa et al.
O
N
Ar
N
O
CHCOCH3
(CH2)7CH3
Ar
CHCOCH3
(CH2)7CH3
Ar
N
O
H
+
+
+
2b
XIV
X
XI
O
N
Ar
N
O
H
Ar
N
O
CHCOPh
ArH
CHCOPh
3
+
(CH2)11CH3
(CH2)11CH3
XII
XIII
XIV
t-Bu
t-Bu
Ar =
t-Bu
Scheme 9. Photolysis of ketones 2b and 3 in the presence of di-tert-butyl peroxide and TTBNB as spin trap.
O
hυ
Ar N O CHCOCF3
+
Ar N CHCOCF3
1a
1b
TTBNB, t-Bu2O2
(CH2)9CH CH(CH2)3CH3
(CH2)9CH CH(CH2)3CH3
XV
XVI
O
Ar N O CHCOCF3
(CH2)13CH3
Ar N CHCOCF3
+
(CH2)13CH3
XVII
XVIII
t-Bu
Ar =
t-Bu
t-Bu
Scheme 10. Photolysis of ketones 1a and 1b in the presence of di-tert-butyl peroxide and TTBNB as spin trap.
O
Ar
N
CHCH2CH2CHOHCH3
(CH2)5CH3
Ar
N
O
CHCH2CH2CHOHPh
(CH2)9CH3
XIX
XX
O
N
t-Bu
Ar
CHCH2CH2CHOHPh
(CH2)9CH3
t-Bu
Ar =
t-Bu
XXI
Scheme 11. Photolysis of ketone 2b (radical adduct XIX) and ketone 3 (radical adducts XX and XXI) in the presence of TTBNB as spin trap.
c
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EPR/Spin-trapping study of free radical intermediates
Figure 4. (a, above) EPR spectrum of the mixture of spin adducts X,XI and XIV, upon irradiation of ketone 2b with di-tert-butylperoxide as initiator and
TTBNB as spin trap. (a, below) Computer simulation using the parameters given in Table 2. (b, above) EPR spectrum after 24 h in the dark. (b, below)
Computer simulation.
Figure 5. (a, above) EPR spectrum of spin adducts XIX and XIV upon photodegradation of ketone 2b with TTBNB as spin trap in the absence of initiator.
(a, below) Computer simulation using the parameters given in Table 2. (b, above) EPR spectrum of spin adducts XX, XXI and XIV upon photodegradation
of ketone 3 with TTBNB as spin trap in the absence of initiator. (b, below) Computer simulation using the parameters given in Table 2.
rise to radicals derived by abstraction of an α-methylene proton Experimental
to the carbonyl. The elusive trifluoromethyl radical has been
Chemicals
indirectly detected for the first time upon irradiation of 1,1,1-
(Z)-1,1,1-Trifluoro-13-octadecen-2-one (1a) was prepared in a
trifluoroacetone (1d) and hexafluoroacetone (1e) in the presence
four-step process from (Z)-11-hexadecen-1-ol, as previously
of hydrogen peroxide and MNP or TTBNB as spin trap. However,
described.[8] 1,1,1-Trifluoro-2-heptadecanone (1b) was obtained
from hexadecanoic acid, TFAA and pyridine,[19] 1,1,1-trifluoro-2-
undecanone (1c) was prepared from decanoic acid as previously
shown.[20] 1,1,1-Trifluoroacetone (1d), hexafluoroacetone (1e), 2-
photolysis of long-chain TFMKs, such as 1a, 1b and 1c, yields
mainly α-carbonylmethylene radicals instead of trifluoromethyl
radical.
c
Magn. Reson. Chem. 2010, 48, 198–204
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E. Rosa et al.
heptanone (2a), 2-undecanone (2b) and tridecylphenyl ketone (3)
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spectrometer at 9.5 GHz (X-band) using 100 kHz field modulation
at room temperature. g-Values of the radicals were determined
with Mn2+ as standard (g = 2.0054). The samples, neat or
dissolved in benzene or acetone, containing a spin trap and in the
presence or not of hydrogen peroxide or di-tert-butyl peroxide
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an Oriel high-pressure mercury lamp (500 W). When necessary,
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[15] K. Torssell, Tetrahedron 1970, 26, 2759.
We gratefully acknowledge CSIC for an Associated Unit UdL-CSIC
fellowship to E. Rosa and CICYT (projects AGL 2006-13489-C02-01,
AGL 2006-12210-C03-02 and PPQ2003-06602-C04-01) for financial
support.
[16] Trifluoromethyl radical was first trapped from photolysis of
trifluoroiodomethane in benzene in the presence of phenyl-tert-
butylnitrone (PBN) (E. G. Janzen, B. J. Blackburn, J. Am. Chem. Soc.
1968, 90, 5909; or in the presence of 2-nitroso-2-methylpropane
(MNP) as spin trap K. J. Klabunde, J. Am. Chem. Soc. 1970, 92,
2427; Trifluoromethyl radical was also detected in gas-phase
by photolysis of 1,1,1-trifluoroacetone in the presemce of PBN
E. G. Janzen, I. G. Lopp, T. V. Morgan, J. Phys. Chem. 1973, 77, 139.
[17] L. R. C. Barclay, D. L. Carson, J. A. Gray, M. Grossman, M. G. Khazanie,
Can. J. Chem. 1978, 56, 2665.
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Magn. Reson. Chem. 2010, 48, 198–204